Accretion, mass wasting, and partitioned strain over the 26 Dec 2004 Mw9.2 rupture offshore Aceh, northern Sumatra

Abstract

The Aceh segment of the Sunda Arc absorbed the largest slip along the 26 December 2004 Mw9.2 subduction earthquake rupture and caused the highest and most disastrous tsunami run-ups. Sea floor disruption marks the frontal active part of the accretionary prism along that segment. An ROV, bathymetric, and seismic survey (R/V Natsushima, JAMSTEC 2-3/2005) revealed pervasive and deeply rooted fracturing along the upper trench slope. Grabens and ejecta clasts (from fluid escape?) along the crest of an anticline ridge coupled with mass wasting and erosional scouring on the slope below it. Specific features of the disruption and its patchy distribution suggest coseismic faulting and folding with gravitational collapse of the steepened anticline forelimb. The same ridge 40 km along strike was pristine. Disruption was also generally undetected by ROV on the Ache Basin side of the forearc ridge, suggesting a minor role for shaking. Only some of the faults in the imbricate and segmented accretionary structure ruptured, but together they may account for 2004 sea-floor rupture along the Aceh segment. This hypothesis accounts for unusual thrust aftershocks and suspended sediment along the frontal active part of the prism. It can also account for the huge tsunami heights in northern Sumatra because faults in the prism are steeper than the megathrust down-dip and because slip may be larger in the prism following prior blind ruptures. Rare but large coseismic deformation of the prism can generally account for the erosional morphology of trench slopes and for al least some ‘tsunami earthquakes’. On the landward side of the forearc high, the West Andaman fault borders the Aceh Basin and branches southward into a fault crossing obliquely the forearc high. Seismic profiles at the bifurcation image a flower structure with strain partitioning and an active dextral pull-apart. This fault system, however, did not release significant strain in 2004.